The film formed from a polyolefin including a linear low-density polyethylene (LLDPE) has various advantages such that it has excellent heat seal properties, it is flexible and tough, it is excellent in resistances to water, moisture and chemicals and its production cost is low. Thus, this film has widely been employed.
It is generally common practice to produce the above polyolefin by the liquid phase polymerization process such as the solution polymerization or suspension polymerization process. For example, LLDPE is usually produced by copolymerizing ethylene with an .alpha.-olefin having at least 4 carbon atoms using a titanium based catalyst (Ziegler catalyst) in the liquid phase polymerization process. Further, in recent years a metallocene catalyst comprising as a catalytic component a compound containing a metal of the group IVB in the periodic table such as zirconium, of which a ligand is a group having a cyclopentadienyl skeleton, has been developed as a catalyst capable of (co)polymerizing an olefin in high catalytic activity.
When the polyolefin is produced by the gas phase polymerization process, the polyolefin can be obtained in particulate form. Thus, it is unnecessary to conduct, for example, a step of precipitating particles from the polymer solution and a step of separating the particles, so that the production process can be simplified. In carrying out the process of gas phase polymerization of an olefin, the olefin in gaseous form is fed into the reactor at a lower part thereof so that solid particles comprising the catalyst and the produced polyolefin are fluidized to thereby form a fluidized bed.
In this gas phase polymerization, the termination of the polymerization after the production of the polyolefin is accomplished by not only terminating the supply of the catalyst and monomer to be polymerized but also depressurizing the inside of the reactor to thereby purge the internal gas from the reaction system, optionally along with replacing the monomer gas by an inert gas or along with feeding a deactivator through a monomer supply line.
However, in a catalytic system of a catalyst having high activity and prolonged life such as the above metallocene catalyst, its catalytic activity is retained for a certain period of time even after the inside of the reactor has been depressurized. Thus, the problem is encountered that the polymerization of the olefin is advanced even after the depressurization and the heat generated by the polymerization reaction causes fusing of the polyolefin remaining in the reactor to thereby invite an agglomeration thereof. When the agglomeration of the polyolefin occurs, not only is the fluidity deteriorated to thereby disenable use of the polyolefin as seed powder but also it is required to withdraw the agglomerated polyolefin from the reactor to thereby bring about an economic disadvantage.
In order to rapidly terminate the polymerization reaction of the olefin in the above catalytic system of high activity and prolonged life, a deactivator is fed in the reactor for terminating the polymerization of the olefin and, in such a case, the infiltration of the deactivator comes to a rate-determining factor. Thus, prior to the polymerization terminating operation, preparatory steps for the termination of the polymerization are effected, such as withdrawing the polyolefin from the fluidized bed reactor so as to reduce the amount of solid particles therein or performing the polymerization of the olefin under such conditions that the polymerization activity is lowered.
However, the practical value of the polyolefin produced through the above preparatory steps for the termination of the polymerization (hereinafter may be referred to as "product produced during the termination period") is very low. Therefore, it is desired to rapidly terminate the polymerization of the olefin without inviting the occurrence of the product produced during the termination period.
Moreover, when the polymerization of the olefin has been terminated by the use of a large amount of deactivator in high concentration, a treatment for removing the deactivator must be added for using the polyolefin remaining in the reactor as seed powder in the subsequent polymerization reaction.
Therefore, there has been a strong demand for a method of terminating a gas phase polymerization of an olefin, a method of initiating a gas phase polymerization of an olefin and an apparatus suitable for use in the above methods, which, especially in the gas phase polymerization of an olefin in the presence of a catalyst of high activity and prolonged life, enable rapidly terminating the polymerization of the olefin without the occurrence of the product produced during the termination period even if the amount of used deactivator is small and enable using the seed powder remaining in the fluidized bed reactor after the termination of the polymerization of the olefin, as it is, in the subsequent gas phase polymerization.